1. Field of the Invention
The present invention relates to an optical data storage medium in an optical data storage device, more particularly, a high density optical data storage medium where the spot size of a laser light focused on the medium secondarily is decreased below the diffraction limit through interaction with a super resolution (SR) material constituting the medium.
2. Description of the Background Art
A primary method to boost the recording density of an optical recording medium is to reduce the spot size of a laser beam in use for recording and reproducing information. When a laser beam of a wavelength λ with a Gaussian intensity profile is focused through an objective lens with a numerical aperture NA, it has a full width half maximum (FWHM) spot size of 0.59 λ/NA at diffraction limit, setting a limit in readout resolution to λ/2NA in terms of a spatial period. Accordingly, in order to achieve a high density for an optical recording medium by use of a diffraction-limited focused light, it is necessary to reduce the wavelength λ or to increase the numerical aperture of the objective lens. However, the increase in storage density by the conventional art has reached a practical limit, as use has been already made of laser light with a shortest wavelength in the visible regime along with a high NA (0.85) close to the theoretical maximum of numerical aperture (1.0).
Numerous techniques have been proposed to overcome light diffraction limit and thereby to accomplish a high density recording and readout. Among them, SR techniques make it possible to reproduce a high density information of a spatial period above λ/2NA by use of a reduced beam spot relative to the one at diffraction limit, which derives from optical changes of various physical origins within a portion of the irradiated area of an SR layer constituting the optical recording medium. These techniques have a remarkable advantage over other techniques in that a high density can be achieved beyond that of diffraction limit, yet on the basis of a far-field optic system of the existing optical recording devices.
Led by a U.S. Pat. No. 5,153,873 which discloses a SR optical recording medium including a single layer nonlinear material, SR techniques have been proposed utilizing various groups of materials such as thermochromic materials, photochromic materials, phase change materials, optical bistable materials, and higher-order nonlinear optical materials.
SR materials utilized in the existing SR techniques may be divided into two different types on the basis of the way that optical transmittance (ratio of transmitted light intensity to incident light intensity) varies with incident light intensity. Examples of the two types are shown in FIGS. 1A to 1B and FIGS. 2A to 2B, where optical transmittance is increased with incident light intensity.
In the case of an SR material as typified in FIG. 1A, optical transmittance changes discontinuously once light intensity or temperature rise due to light absorption exceeds a threshold intensity or a threshold temperature with increasing incident light power. FIG. 1B schematically illustrates how incident light may change in its intensity profile, upon transmission through the SR layer, with increasing incident light power (P2>P1). At a lower power (P1), optical transmittance is uniform and low (denoted by shorter arrows) along the Gaussian intensity profile of the incident light but, at a higher power (P2), the intensity profile bears a portion around the center where light intensity exceeds a threshold value to yield a higher optical transmittance (denoted by longer arrows) than that of the peripheral portion (the central region of the super-resolution layer with a different optical transmittance from the rest is denoted hereafter as an aperture). The size of the aperture increases with increasing incident light power.
An SR material of this type is structurally and chemically discontinuous at a threshold intensity (temperature), undergoing a transition between phases of different refractive indices and extinction coefficients. Examples include thermochromic, photochromic materials of either organic or inorganic nature, phase change materials and so on. As for the phase change materials in particular, examples include materials undergoing solid-solid phase transitions such as AgZn and CuAlNi; compound materials undergoing decompositions such as AgO; materials undergoing solid-liquid phase transitions such as chalcogenide alloys like Ge—Sb—Te, In—Sb—Te and pure metals like In, Te, Bi, Pb, Sn, Sb; and materials consisting of dielectric matrices with dispersions of the aforementioned materials having solid-liquid phase transitions.
In the case of an SR material of the other type, optical transmittance tends to increase not discontinuously but gradually once light intensity exceeds a threshold value with increasing incident light power, as shown in FIG. 2A. In relation to this, FIG. 2B schematically shows how incident light may change in its intensity profile, upon transmission through the super-resolution layer, with increasing power. In FIG. 2B, the arrows indicate the relative magnitudes of optical transmittance according to the local intensity of incident light along each intensity profile. Compared with FIG. 1B, it is noticed that, at a higher power, the intensity profile bears a portion around the center where light intensity exceeds a threshold value, yielding a gradual increase of optical transmittance with light intensity.
An SR material of this type has neither structural nor chemical change beyond threshold intensity but undergoes a gradual change in optical characteristics as light intensity increases beyond the threshold value. As a typical example, there exists a group of materials, called self-focusing materials, that have intensity-dependent capability of focusing light due to third order nonlinear optical effect. Besides, there are saturable-absorption materials showing a gradual change in optical transmittance over a wide range of light intensity beyond threshold value.
Regardless of its type, an SR material is supposed to satisfy the following requirements primarily. First, changes in optical properties must be sufficiently large across the threshold intensity (temperature). Second, said changes should occur rapidly and reversibly with changing laser intensity. Third, a laser power should not be too high to reach the threshold intensity (temperature). Fourth, an SR material should have a high endurance against repetitive laser irradiation for use in read-only-memory (ROM) disks, write-once-read-many (WORM) disks and rewritable disks as well.
SR technique has been considered to have a high potential for ultrahigh density optical data storage beyond optical diffraction limit without sacrificing a critical advantage of the present optical disk technology, i.e. removability. Except for magneto-optic disks, however, SR technique has rarely been materialized due to unsuccessful development of a material satisfying all the aforementioned requirements.
As for the first two requirements which are especially critical to SR capability of reproducing information below a resolution limit, the most promising results have been obtained mostly with materials yielding large changes in optical properties due to phase transitions of either melting or decomposition, hence SR materials of the first type. Each of these materials, however, has led to limited readout cycles as in the case of ‘premastered optical disk by super resolution (PSR)’ utilizing the melting transition of a chalcogenide GeSbTe alloy or limited applicability to write once read many type disks as in the case of super resolution near-field structure (Super-RENS) disks utilizing the oxide decomposition of either AgO or PtO. For an SR technique to find its use in ROM disks and rewritable disks in particular, development of a new class of SR materials appears to be called for that can maintain a high SR capability during a long cyclic readout and/or writing operation. To fulfill such a requirement, materials are not supposed to owe their SR capability to phase changes, especially of the kinds accompanying structural changes.
Accordingly, there is an urgent need for the development of a practical SR material that can provide a high endurance against repetitive laser irradiation as well as with a high carrier to noise (C/N) ratio and is thus applicable to ROM disks, WORM disks and rewritable disks.